【哈希表】Longest Consecutive Sequence

题目：leetcode

Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,

Given [100, 4, 200, 1, 3, 2],

The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

分析：

用一个哈希表，记录一个数是否已经被遍历，避免重复。

leetcode的测试用例没有考虑输入数组包含int类型最大最小值的问题。如输入[0x7fffffff,0x80000000]，理应返回1，但如果没有考虑int类型最大最小值的问题，会返回2（即认为 0x7fffffff和0x80000000是两个相邻的数）。

int longestConsecutive(vector<int> &num) {
        if(num.size()<=1)
        return num.size();

        unordered_map<int,bool> n;
        for(auto &i:num)
        {
            n[i]=false;
        }
        int res=1;
        for(int i=0;i<num.size();i++)
        {
            if(n[num[i]]==true)
            continue;
            int length=1;
            if(num[i]!=0x80000000)
            {

                int number=num[i]-1;
                while(n.find(number)!=n.end())
                {
                    n[number]=true;
                    number--;
                    length++;
                    if(number==0x80000000)
                    break;
                }

            }
            if(num[i]!=0x7fffffff)
            {
                int number=num[i]+1;
                while(n.find(number)!=n.end())
                {
                    n[number]=true;
                    number++;
                    length++;
                    if(number==0x7fffffff)
                    break;
                }
            }
            res=max(res,length);

        }
        return res;
    }